This invention relates to a formulation of mineral fibers suitable as acoustical and thermal insulation. One useful application of the mineral fiber formulation in accordance with preferred embodiments of the invention is in the fabrication of acoustical ceiling boards. The material is also suitable for use as loose fill insulation in, for example, attics and walls.
The mineral fibers must be stable enough to withstand the processing involved in making the thermal and acoustical insulation products, and yet still exhibit acceptable levels of solubility in biological systems.
The mineral fibers according to the embodiments of the invention disclosed in this application are produced principally from slag. Slag is a non-metallic vitreous material consisting primarily of silicates and alumino-silicates of lime, magnesia, and other bases. The four principal oxides of typical slags are CaO (25-50% by weight), Si0.sub.2 (25-50% by weight), MgO (2-25% by weight) and Al.sub.2 O.sub.3 (5-20% by weight). Unless otherwise specified, percentages used throughout this specification are percent by weight. Slag is formed as a byproduct from metal extraction operations, and may therefore be recovered from iron blast furnaces, open hearth steel furnaces, and copper, lead and precious metal smelters. Slag is generally viewed as a waste stream with little utility and thus as a commodity with a relatively low value. It can therefore provide a very cost-effective raw material resource if economically processable into a commercially viable product.
Slag has proven useful in the production of certain man-made vitreous fibers, sometimes called "mineral wool" or more specifically "slag wool." These terms are used interchangeably in this application and refer to the matted, wool-like fibrous appearance of the product. Man-made mineral fibers are made by melting a suitable slag-based composition to form a "melt" and then blowing or spinning the molten material with sufficient energy to cause it to be mechanically transformed into fibers. The technology and knowledge base for such fiber production is well established. Records indicate the production of slag wool as early as the 1840's in Wales, and 1873 in the United States.
In a standard mineral wool fiber, alumina and silica form the principal mechanical strength and attachment elements. The other elements act as fluxes affecting the melt temperature and viscosity profile of the formula. When the viscosity of the material is low and the melt flow rate is too high the fibers will be thin and excessively large portions of the material will fail to form fibers. Instead, some of the melt forms clumps or balls of vitreous material, sometimes referred to as "shot." If the melt viscosity of the molten material is too high, then the resultant fibers will be thick and brittle and in addition there will be too high a proportion of shot.
Both theoretical and empirical research have determined that certain materials have an effect on the properties of mineral wool, and that varying the percentages of certain materials in particular ways can effect the processing parameters and properties of the end product in either a beneficial or detrimental manner.
For example, Calcium Oxide (CaO) decreases the viscosity ofthe melt, and increases the slope of the temperature viscosity curve. An increase in the slope of the temperature viscosity curve is detrimental to the properties of the end product. Generally, the slope of the temperature viscosity curve should be relatively flat. If viscosity rises too quickly with temperature, then the process control is difficult to achieve.
Aluminum Oxide (Al.sub.2 O.sub.3) increases the viscosity of the melt, and beneficially decreases the slope of the temperature viscosity curve. Fiber ductility is also improved.
Magnesium Oxide (MgO) also increases the viscosity of the melt, and beneficially decreases the slope of the temperature viscosity curve.
Iron (III) Oxide (Fe.sub.2 O.sub.3) decreases melt viscosity.
Silicon Dioxide (SiO.sub.2) increases viscosity of the melt. It is added to balance the formula and vary the solidification point of the fibers.
The approximate composition by weight of the four principal oxides that make up most commercially available slag wool fibers are:
Silicon Dioxide (SiO.sub.2) 38-45% Calcium Oxide (CaO) 28-38% Aluminum Oxide (Al.sub.2 O.sub.3) 9-14% Magnesium Oxide (MgO) 2-12%
To achieve a suitable viscosity for good fiber formation SiO.sub.2, often in the form of gravel or sandstone, is compounded with the slag. A measurement often used to evaluate a molten mineral wool-forming composition is its "A/B Ratio." "A" is the sum of the SiO.sub.2 percentage and the Al.sub.2 O.sub.3 percentage. "B" is the sum of the CaO percentage and the MgO percentage. Slag wool fibers with an A/B ratio of 1.0 (i.e., equal percentages of A and B) have performed well in some biosolubility tests, and not well in others. An A/B ratio of 1.0, however, is quite low, and the viscosity of melts having such A/B ratios tends to be too low to achieve good fiber yields with car-type spinning systems. Slag wool with A/B ratios in the range of 1.2 to 1.4 are more typical, and these have not performed well in biosolubility tests.
Some other types of mineral fibers produced from raw materials other than slag have been shown to be thermostable and suitable for producing acoustical ceiling board products, and have performed well in biosolubility tests. These fibers are typically formulated such that they either have very low Al.sub.2 O.sub.3 percentages (&lt;4% by weight) or very high Al.sub.2 O.sub.3 percentages (&gt;18% by weight) and low CaO percentage (&lt;29% by weight). These formulations cannot be readily achieved using slag as a base material due to the inherent chemical composition of slag.
Biosolubility is generally considered a beneficial quality in products such as mineral wool. Biosolubility does not refer to complete dissolution of the fibers within a biological system. Rather, biosolubility refers to the ability of a biological organism to attack, weaken and ultimately eject the fiber from the body. Any type of fiber of a certain size can become trapped in the lung as a result of inhalation of air-entrained fibers. These fibers are detected as foreign to the organism and attacked by lung fluid (which has a pH of about 7.4) and also by macrophages. Macrophages are an important part of the body's defense mechanism. Macrophages are large phagocytic cells found in the spleen, lymph nodes, liver and many other body tissues. They develop from monocytes and are characterized by a horseshoe-shaped nucleus and nongranular cytoplasm. Macrophages have an internal pH of about 4.5. If the macrophages are smaller than the fiber, several of the macrophages collectively engulf a fiber. The macrophage and lung fluids chemically attack the fiber, weakening it. The fiber can then be broken into pieces which are carried away by the macrophages. The macrophages migrate to the trachea where they are trapped in mucous and ejected from the body by being coughed-up in phlegm, or by being swallowed and eliminated through the digestive tract. Biosolubility may be evaluated via animal tests such as those described in EU Directive 97/69/EC or by in-vitro testing in simulated biological fluids, such as a 4.5 pH Gamble's solution. In general, to pass the EU Directive 97/69/EC tests requires a solubility greater than 1000 Kdis (measured in ng/cm.sup.2 * hr).
The invention according to this application results from a novel appreciation and application of the interrelationships between several of the constituent parts of slag, and how those constituent parts can be manipulated to produce a mineral wool which has desirable and commercially valuable properties while also having a relatively high degree of biosolubility.